Most people will have heard the term “constellation” and will know that it relates to patterns of stars in the sky.

Many people will probably think of astrology and the “Signs of the Zodiac”, for example Aquarius and Pisces.

But how many people know what the constellations actually are?

The star constellations are really just a fairly arbitrary way of dividing up the night sky, in a memorable way.

But arbitrary or not, the constellations are very useful and help observers to find their way around.

Dividing the sky up in this way, probably started as long ago as humankind itself.

Early people certainly drew recognisable shapes representing star patterns and also began to associate these shapes, with Gods or important legends and stories.

The constellations became somewhat more precisely defined with the early Greek astronomers, who classified a total of 48 star patterns.

At this time, the notion of the “Zodiac” stars also began. The Zodiac is the 12 star patterns or constellations, that the Sun appears to pass through, during the 12 months of the Earth year.

Hence the idea of each constellation being associated with a specific month of the year.

Interestingly, there should probably be thirteen signs in the Zodiac, because the Sun does actually pass through the constellation of Ophiuchus, as it is now defined!

There is one very important point to make about the constellations. Each constellation is not a real grouping of stars – it is only the appearance of a group, from our viewpoint here on Earth.

In fact, the stars in any particular constellation are at greatly different distances from Earth. So if we were to see them from a different position in our galaxy, they would not look like a group at all.

The constellations as astronomers know them today, were defined properly in the 1930′s, by the International Astronomical Union (IAU).

There are now 88 constellations. They vary in size, but generally, each constellation extends beyond the recognisable pattern of naked-eye stars (called an asterism) into the surrounding sky.

So for example, the constellation of Ursa Major contains the highly recognisable shape of the “Plough” or the “Big Dipper”. This is called an asterism.

The Plough asterism is made up of seven stars that appear bright to us on Earth. But the constellation of Ursa Major is much larger and contains many, many more stars and indeed other galaxies, that we can observe.

Taken together, the 88 constellations map out the entire sky into 88 areas. This is very useful to astronomers, when they want to describe where an object may be observed.